Synchronicity
by chaconneviola
Summary: It began as an experiment of observation- a portal to another universe, a parallel universe. But the problem with doors is that they open from both sides. Avengers and Alt!Avengers, plus slight appearance by Reed Richards. Also Loki. Reviews welcome!
1. Controlled Experiment

Hey guys! This is technically my second fan fiction (since I decided to publish a Dr. Who story which I am totally NOT pulling a shameless plug for), and it's done for fun & giggles. I hope you enjoy it! Reviews are always welcome. -chaconneviola

* * *

"Ever wonder who we'd be if we weren't, y'know, us?" Tony Stark flipped his screwdriver in the air, caught it, and twirled it absent-mindedly.

Across the lab, Bruce Banner raised his head and gave his friend a quizzical look.

"But if I'm not me, who am I?" he quipped.

"Exactly. And Pepper says I'm hard to follow at times." Tony tossed the screwdriver onto the workbench—one of the workbenches—and dusted his hands together. "It's been bugging me ever since the run-in with Evil Spidey."

"Venom," Bruce corrected him.

"Yeah, sure. Same guy, same powers, same… basic charming personality, but you put him in the wrong suit…" Stark shook his head. "An enduring testament to the power of good fashion."

Bruce raised his eyebrows.

"So this is why you upgraded your suit's security. Again."

"I may not be a Boy Scout, but I'm always prepared. Anyway, I happened to run into Reed Richards at the convention last week—y'know, the convention you wouldn't attend—"

"The convention I _couldn't _attend because General Thaddeus Ross was presenting a—"

"Excuses! You could have taken it down. You need to quit underestimating yourself, big guy. And quit trying to change the subject. I saw Reed Richards at the convention, and you wouldn't guess what he's been working on."

Bruce groaned and kept his eyes on the interface he was re-working.

"He finally got his 'parallel universe' machine working?"

"How'd you know?"

Bruce shot him a quick glance over his shoulder.

"You forget I tried to de-radiate myself while I was on the run. I got fairly close to the fringe and crackpot science community. Even saw your name pop up a few times."

"Was that supposed to be a jab? 'Cause that was pretty pathetic. Wasn't even a poke, really. Yeah, Richards is giving his quantum computer a whirl on Tuesday. I thought…" he shrugged. "We might like to have a look at it?"

Bruce sighed and pinched the bridge of his nose.

"You know how I feel about quantum indeterminacy and casuality."

"Don't think of it as casuality. Think of it as… synchronicity," replied Tony. "Parallel universes aren't necessarily intertwined or even linked."

"They are if you observe them!" Bruce snapped. "Or they become… linked… altered…" He shook his head. "Richards is a brilliant guy, and I have a lot of respect for him. But sometimes there are things you shouldn't mess with. Guess I learned that the hard way." He ended with a self-deprecating laugh.

Tony scowled. He hated it when Bruce made remarks like that—like a stealth put-down, but a put-down to himself. Was that even possible?

"Well, I like to think that every system is worth studying, and every source of data is valuable," he said. "Just think about it. You're a biophysicist—can you imagine getting a few simple cells from a parallel universe? A few molecules? Even a couple loose hydrogen ions. Think about how much we could learn—it's literally the information of the cosmos, and it's just sitting there waiting for someone to come look at it."

Bruce didn't move, just sat there and looked at him with hungry eyes. Inwardly, Tony smirked. _Bingo. Get him onboard and make him feel all proud of himself as a genius. Way to go, Stark. _

"Well," Bruce said slowly, "it would be… informative… to run a few tests…"

"Informative?" Tony snorted. "It would be the breakthrough of the century."

"Wait, what?"

Both Tony and Bruce turned to see Steve Rogers, holding a half-eaten donut in one hand and looking confused and slightly lost. Behind him, Thor was diligently opening two Pop-Tarts at once.

"What's the breakthrough of the century? A new vaccine or something?" Steve asked.

"Oh…" Bruce said. "It's, um…"

"It's a bit hard to explain, Captain Responsible," Tony said, stepping in front of Bruce and invading Steve's personal space. "Theoretical physics. Parallel universes. Don't think you'd be interested."

Steve crossed his arms and frowned.

"Don't talk down to me, Stark."

"Parallel universes," Thor put in, licking his fingers. "Friend Stark, I am greatly intrigued. Are you venturing into another of Yggdrasil's branches?"

"Uh… branches, sure, let's go with that," Tony said, and opened his mouth to explain further. Unfortunately, Thor did something extraordinary—he cut off Tony Stark.

"Excellent!" he boomed. "I have long desired to show you the nether realms. And you too, Dr. Banner, although I fear you must need your green beast to travel with us on glorious—"

"Uh, no," said Tony, but Thor would not be stopped.

"—Niflheim is wondrous this time of year, and we are sure to meet with hospitality unparalleled by any—except yours, Man of Iron—"

"Uh, _no," _repeated Tony, wondering why nobody was listening to him.

"—hunting in Jotunheim! And Odin would view it as a personal favor if we put down the yearly Jotunn insurrection; they are worthy foes and will make for glorious battle—"

"Thor, NO!"

Thor stopped, looking slightly hurt. Tony rolled his eyes and sighed.

"Okay, look, we're not going into another realm. It's like… Yggdrasil is a tree connecting the nine realms, right?"

"Of course!"

"Well, this is like… another tree that grew next to Yggdrasil, and the branches are almost touching—see, like this—" Standing on tiptoe, Tony traced a crude diagram of two trees on his digital screen. "There. And we're going to take a peek into the Midgard of the other tree."

"A second Yggdrasil?" Thor looked puzzled. "This is dangerous folly, friend Stark. Heimdall has told me nothing—"

"Thor, just stop," Steve interrupted. Biting his lip, he turned to Bruce. "Is he for real?" he asked, pointing at Tony.

Bruce nodded.

"Okay," Steve said simply. "A… parallel universe. I've taken in floating words, earplug music, and magical food dispensers. I'll deal with a parallel universe when I see it."

Tony glanced up at that.

"Oh. No. Bad. Bad idea. You're not coming. You're not—"

* * *

Crowded between Thor and Bruce in the jet, Tony scowled and crossed his arms.

"I hate you, Rogers," he stated.

"So you've said."

"Don't be so hard on him," Natasha said coolly, from the copilot's seat. "He did the nation a great service, alerting us to a new potential threat."

Up in the cockpit, Steve's shoulders straightened a little. Tony ground his teeth together.

"Oh, and that's all that counts, isn't it? An item's value as a potential threat," he sneered.

"Remind me of your business nickname again," said Clint.

"Keep on like that, I'm gonna take back those new arrows, birdie boy," Stark muttered.

"Besides," Bruce put in helpfully, "it really is a good idea to have a pair of military eyes on the experiment. Just in case something goes wrong…"

"I'm going to be doing the experiment; how could it possibly go wrong?" Tony snapped.

Everyone in the jet exchanged glances, but said nothing.

"Did you not fill our dining area with smoke last week?" Thor said. "Twice?"

"Shut up."


	2. Complications

"Tony! So glad you could make it! And, er, you brought guests. Fantastic!"

Natasha almost rolled her eyes at that. Reed Richards was pretty much what she had expected—handsome, charismatic, fatherly, brilliant. Almost too good to be true. Sure, there was a touch of grey in the perfectly trimmed brown hair, and he didn't stand right—compensation, she guessed, for having his skeleton turned into the biological equivalent of Silly Putty—but overall, he had it together. The perfect hero.

She didn't trust him. Nobody was that perfect.

"Reed, buddy." Stark shook his hand, and couldn't resist pulling back suddenly to watch Richards' arm elasticize on instinct. Natasha watched Richards' face closely. Flash of surprise, then delight. He liked the attention. "My team. Steve Rogers, Natasha Romanov, Clint Barton, and, uh, Thor."

"I am honored to make your acquaintance!" Thor boomed, clapping Richards on the shoulder. "I understand you will show us a second Yggdrasil."

Richards looked confused for a moment, but smiled and shook his hand.

"Oh! Don't forget your guest passes," he said, passing a small, clip-on nametag to Stark and Thor. There was a flash of something in his face, and then it was gone. Natasha watched him closely as he shook hands with Clint Barton—hint of distrust, general politeness—and Steve Rogers—admiration, respect, tinge of jealousy. And Bruce Banner.

Reed Richards' face went carefully neutral.

"Dr. Banner. Pleased to meet you. I enjoy reading your work."

Banner dropped his eyes, muttering something polite, and moved quickly past Richards to Stark, fumbling with his tag. He usually did when people made oblique references to the Hulk. Natasha shook hands cleanly and professionally, and followed her team. Partially out of habit and partially out of re-awakened paranoia, she did a visual sweep for cameras and security devices as they moved through the lab. Or mansion, really. She'd thought Stark's lab was large and expensive.

Natasha couldn't help noticing the number of cameras, tripwires, and sensors in the laboratory. Richards wasn't the paranoid type, and the other members of the Fantastic Four lived at the building (where were they, anyway?), so the security measures were… disconcerting. Natasha walked briskly and professionally and mapped out paths of escape in every room they passed.

"Here she is," Richards said at last, stopping in front of an enormous piece of equipment that looked… well… like something from a science-fiction movie. Natasha registered a twenty-foot screen and several control boards. The rest was a mystery. Stark and Banner were already playing with the control panels, looking like kids in a candy shop. She heard the words "teraflop" and "quantum operator" being tossed around.

"So, this is it," Rogers said, with a hint of disappointment. "The world's… biggest computer."

"Yep. Pretty impressive," Natasha said.

"It looks like something from…" Rogers shook his head. "Flash Rogers or something." He sighed and put his hands in his pockets. "So this is today's technology."

"No, this is Reed Richards' technology," Barton said. "I'm still having trouble getting my head around it myself. Parallel universe." He shook his head.

"And we don't know what could come out of there, so let's get our weapons ready and stand back when they fire it up," Natasha said.

"Come out of there?" Rogers said blankly. "I thought it was like—like a telescope."

"Not quite. It's really more like… a window," Natasha said. "I've been talking to Bruce—something about Heisenburg and determinism and our photons connecting with their atoms. I don't understand the technical terms. But I do understand there's a risk. We're making a connection to another reality—for now, it's observation, but, as with any connection, there's always the possibility…"

"The Tesseract," Clint said, bitterly.

"Not quite. But…" Natasha stepped back and checked her gun. One of her guns. "It doesn't hurt to be ready."

"Explain to me again why this metal…" Thor's eyebrows went up, and he hunted for a word. "This metal…"

"Computer," Reed Richards supplied.

"Why it is a 'she?'" Thor asked.

"Because she's gorgeous and powerful and probably gets hot when Reed's around," Tony shot in, before disappearing under the mainframe again. "Bruce, buddy! Look at this. Look. At. This."

Thor laughed. The Man of Iron and Dr. Banner were bound together—he sensed they were brothers of the battlefield, as he felt among his Warriors Three. They met over bewildering metal objects, not scores of slain enemies. It was the same.

But it had been a long time since he saw either one of them so… happy.

"This computer, she serves as Heimdall does in Asgard," he said, half-stating and half-questioning. "To let you see where you cannot."

"That's the theory, buddy," said Stark.

Suddenly, Richards' head appeared over Thor's shoulders.

"Ready to take a look?" he asked.

"Oh, yeah," Tony said. In literally seconds, he, Bruce, and Thor were pressed up against the screen (and only later would he wonder why Cap and the assassins were hanging back), staring at the blank monitor with eager eyes. There was a low whir as the machine hummed to life, and the room slowly dimmed.

"Come on," Tony said under his breath. "Come on come on come on. Show us your secrets…"

BOOM! The light exploded out of the console, and for a moment, nobody could see anything. Tony closed his eyes reflexively, noting the number the light was doing on his inner ear—wait. Light shouldn't affect inner ear function. It sure felt like he was getting vertigo, though. Maybe he shouldn't have stood so close to-

* * *

Blurrily, Tony opened his eyes.

"Okay, Reed, you got me," he said, exhausted. He was on his back and it was dark, he knew that much. "Great trick, top stuff, now can we get back to firing up the computer and checking out the—" Suddenly, the room came into focus, specifically the face leaning over his. "Reed, buddy. Hey, wait a minute, you're not Reed. You're not my—oooooh."

"Careful," Reed said. Tony winked at him—he hoped it was a wink—and sat up slowly.

"You're a devious little bastard, you know that?" he said. "Nametags… you chipped the badges and used them as focus points. So was it you, or my Reed, or both of you working together?"

Reed chuckled. He looked almost identical to the Reed Richards Tony Stark knew and bantered with, but his hair had gone completely silver, and he wore a very dark blue turtleneck instead of the customary Fantastic Four uniform and lab coat.

"Intelligence. I always liked that about you, Dr. Stark."

"Mr. Stark. Don't call me doctor unless you're willing to wear the nurse uniform—and, uh, not even then, buddy. No offense."

"He's not your buddy," came Natasha's voice. Craning his head, Tony made out Natasha and Clint perched on the edge of a large crater. A crater, really? In the lab? Then Tony's brain focused, and he recognized bent metal edging a very large—very Hulk-shaped—hole in the wall. Oops. He made a quick mental note to pay for that later. Assuming he was still a billionaire over here.

"No? Is this the evil doppelganger? Please tell me we're not gonna get into a Kirk/Spock standoff," Tony said.

"I'm not an evil doppelganger, Mr. Stark, and in reply to your question: both myself and… myself… designed the experiment," Reed said smoothly.

"Devious," Tony said, nodding. "We'll have a little talk later. Okay… Natasha, Clint… there's Thor… where's Bruce?"

"He transformed," Clint said. "Stress of breaking through, I guess. Cap went after him."

"Not before he wrecked the computer, though," Natasha snapped, nodding at a hunk of twisted, smoking metal that must have looked like a science fiction prop at one time.

Oh. God. No. Tony actually couldn't think for a moment. He wrecked the computer? Reed Richards' state-of-the-art, hadron-sensitive quantum computer? He wouldn't.

"He didn't!"

"I'm afraid he did," Reed said. "Don't worry about it, though; I'll have it repaired in under two weeks."

"Two weeks?" Thor scowled, a thundercloud building on his brow. "So we are trapped here for two weeks?"

"Thor, buddy, relax," Tony put in. His head still felt like a sledgehammer was beating inside it. "I'm sure Reed has Pop-Tarts and a spare bed. If not, I'll find myself over here—I assume I exist, besides me, right?—and crash at Stark Tower. Hey Reed! I'm alive over here, right?"

"Ah… Dr. Stark is a good friend of mine," Reed said diplomatically.

"Tony Stark, right? Tell me my dad didn't name me Bertram or something idiotic like that."

"Yes, that's right," Reed said. Suddenly, Natasha was right next to him. Either she was moving faster than usual, or Tony was starting a mild concussion.

"You're not telling us something," she snapped. "Listen to me, Dr. Richards. I hate being lied to. I hate being kidnapped. I hate being 'volunteered' for experiments. You wanna tell me what's going on here?"

Reed didn't move for a moment. Then he swallowed and looked down.

"Of course, Miss Romanov." He moved back, undulating, a few feet. "Our two universes are the most similar I—and your Reed Richards—have discovered. We've been searching for about six months. Reed made contact with me about two weeks ago and suggested a—collaboration of sorts. You see, our timelines are very similar. Loki attacked New York about five months ago, and Fury responded by assembling a unique team—the Avengers. Like your Avengers, our members are Tony Stark, Natasha Romanov, Clint Barton, Bruce Banner, and Steve Rogers, but…" he shifted uncomfortably. "Thor returned with Loki two months ago. Apparently Odin wanted to punish Loki through "redemption"—living on earth as a human for one year."

"You're kidding, right?" Natasha said.

"Yes, well, one doesn't reason with a god," Reed said ruefully. "Loki escaped from Thor's custody, ran amok in Vegas, killed fifty-eight people—the body count is still rising—and, as of yesterday, regained at least some of magic, despite Odin's promises. Thor is missing."

"So why send over the whole team?" Tony asked. "I mean, I could understand snatching Thor as a replacement goldfish, but why us?"

Reed twitched.

"It's… complicated," he said.


	3. Unexpected

BOOOM! Hulk's big fists smashed through the puny wall, and he skidded to a stop inside the library, stumbling a bit over his own feet. People were screaming and running. Stupid people! Hulk not hurt them! Hulk only want smash walls. Wall hurt Banner. Hurt Hulk!

Pain exploded across Hulk's back. Someone shooting Hulk! With a feral roar, Hulk spun and swatted the hovering helicopter away.

"LEAVE HULK ALONE!" he roared.

Then something hit him in the back. Hulk looked down, ready to smash. But he smiled instead.

"TONY!"

"Yeah, it's me, big guy," came Tony's voice, through the helmet.

"TONY'S VOICE FUNNY!"

"Okay, that's a new one. Hang on a mo, Hulk, we're almost out of the city."

Hulk shouted with laughter. Tony take Hulk away, to quiet green place, take Hulk away from guns and screams and walls that hurt. He tumbled to a stop in the field and sighed. Hulk tired.

* * *

The hardest part about waking up after an "incident," Bruce thought, was trying to remember what had happened just before said "incident." He chuckled groggily at the thought—no doubt Tony would make an alcohol-related joke, if he could hear—and sat up slowly.

"Whoa, steady there, big guy," a voice said. "You nearly broke Harlem again—nearly, I said nearly, there's little damage done. Well, relatively little. Relatively little for you."

"Wha…?" Bruce shielded his eyes against the sun. Late afternoon, he'd guess. "Where… a field?"

"Yeah, I pulled some strings, bought this last quarter. Just in case you needed to, y'know, crash here for a little while."

"Oh." Bruce turned, squinting. "Who are you again? I mean, I know the armor, but the voice…" Bruce stopped, staring at the armor's torso. That wasn't right. "Whoa."

"Who am I…?" Iron Man tilted his head. "You don't recognize me. Must have hit your head harder than I thought." The helmet retracted to reveal a very beautiful, very curious, very female Iron Man. "I'm the guy you saved, remember? Gave you a job, put you up in my Tower? Ringing any bells?"

Bruce squinted, still trying to reconcile what he was seeing and what he knew.

"…Tony?" he said finally.

"There we go! Toni… last name?" she prompted.

"Toni Stark?" Bruce shook his head. "You've gotta be... I mean, I just... I really went through the looking glass, didn't I."

* * *

Steve Rogers broke into a light jog, headed out of New York City. Following the Hulk hadn't been hard; the green giant tended to leave broken buildings, not breadcrumbs. He hadn't counted on other-Tony flying in and spiriting Hulk away. Still, it made sense. And he'd been lucky enough to spot the beginning of Iron Man's descent; unless Tony had radically altered course and flown away low to the ground, Steve would be able to estimate the landing site.

Along the way, he couldn't help noticing other damage. Overturned cars, partially-burned buildings, hundreds and hundreds of shattered, taped-up windows. It was too old to be the Hulk's and too new to be the Chitauri's damage. Steve frowned, shook his head, and kept running. He'd have to get the official story from someone.

Then he realized that a new universe meant he'd be re-learning references and technology all over again. For a moment, Captain America was very tempted to swear.

And he needed to take a cab. Even running at a fast jog with a superb, perfectly fit body, it would take another thirty, forty-five minutes tops to get out of the Big Apple. Reluctantly, Steve stumbled to a stop and whistled for a cab.

"Hey," he asked, dropping into the seat. "Just drive. I'll show you where."

The cabbie, an older man with a white moustache, colorful Hawaiian shirt, and thick square glasses, shrugged.

"It's your money."

"Yeah," Steve said. "Look, mister, I'm new in town."

"Sure you are." The cabbie laughed. "I've seen your picture on the T.V. You're famous, mister! A real American hero."

"Yeah, okay. Just… play along with me. Remind me what caused that damage…?"

The cabbie whistled.

"You don't remember? That was the last big fight here. Loki and the Fantastic Four. I was right there when he killed Sue Storm. You know that? Right there, in my cab, and I saw—"

"What did you say? Sue Storm's _dead?"_

"Well, yes, sonny. The funeral was last only last week. I haven't seen something like that since the funeral of Princess Diana. Say, you sure you're okay? You want me to take you to a hospital?"

"No. No, here's fine," Steve said abruptly. "Just keep driving."

Steve's stomach knotted as they passed from the city limits into the green liminality. Sue Storm dead. He'd never met her personally, but he'd seen the pictures. Celebrity glamour shots, her with Reed Richards, her outlined in the blue sky, hovering and semi-transparent, her turning at an impossible angle to deliver a force-field punch to an artificial intelligence experiment gone wrong—

And Reed Richards. The perfect, benevolent, unshakable Reed Richards… he swallowed hard.

"Pull over here, please."

* * *

"...no kidding. So this isn't gonna make things weird, is it? I mean, between me and you. We're still good, right? Science bros forever."

The voices drifted over the fields and Steve raised his eyebrows. Bruce and a girl? What happened to Iron Man?

"Well… honestly…" Bruce sounded more than a little flustered. "I don't know you like Tony—er, my Tony. I mean, you must have had… vastly different experiences… I'd have to get to know you first, you know, as yourself. Not… not my Tony. But, um, no. I don't think it will cause problems…"

"Bruce, my man," the woman returned, slapping him on the shoulder. "You're solid as ever. Oh. Hel-lo, Captain."

She stood up and turned around, and Steve took in two things very quickly. One, she was wearing a female version of Tony Stark's red-and-gold armor. Two, she was beautiful. As in, drop-dead beautiful. Short, dark hair framed a face which, honestly, would have looked good on the cover of any magazine. Dark, roguish eyes with thick lashes and—

"Uh," he said.

"My thoughts exactly when I first saw her," Bruce said, coming to his rescue. "Er, Captain Steve Rogers, meet… Toni Stark, this universe's version of Iron Man."

"Iron Woman," she corrected him. "And judging by the surprise on your face, Captain, I'm guessing you're a visitor from another world like Brucie here and not my fiancé. Still gorgeous though, if it makes you feel better." A devilish gleam leaped into her eye. "On second thought—better make sure."

Before Steve could do anything, she'd crossed over to him, grabbed his head, and pulled him into a long, altogether too strong kiss. When she let him go, Steve staggered back, spluttering and wiping his mouth.

"What the—"

"Awww, Steve," Toni grumbled. "Two universes in a row, and you still kiss like a girl."

"Ma'am! Please!" Steve protested, his face bright red.

Bruce coughed, also blushing.

"We should—be going," he said awkwardly.

"Right. You need a shirt, he needs a hot date." Toni paused for a moment, brightening up. "Hey, Steve Two, you wouldn't be open to—"

"_No."_

"'Fraid you'd say that," Toni sighed. "You have no sense of imagination. So, boys, Brucie here says there's been a mix-up with Reed Richards and his computer. Which, by the way, is gorgeous and amazing and I have no problem with the computer as such whatsoever, capiche? Great. So we head back to the Baxter Building, pick up the rest of the Avengers, maybe do some boring press or official stuff with Fury and Richards, and crash back at Stark Tower. Unless, of course, you have somewhere better to go."

"Uhhm." Steve was still apparently too tongue-tied to answer.

"That would be great," said Bruce.

"Awesome. Meet you back at Richards' lab in ten."

With that, she took off into the sky, blazing. Bruce and Steve exchanged looks and turned to hail a cab or borrow a motorcycle, whichever they found first.


End file.
